In mobile communications, a radio communication apparatus fails to properly receive a radio signal to be received if the reception frequency thereof differs from the frequency of the radio signal. Accordingly, a radio communication apparatus used in mobile communications is equipped with AFC (Automatic Frequency Control) so that the reception frequency thereof can be adjusted to the frequency of a radio signal to be received (see, e.g., Japanese Laid-open Patent Publications No. 2006-349587, No. 07-74726, and No. 2002-26769).
Meanwhile, in cases where a radio terminal moves at high speed near a radio base station, for example, where a Shinkansen bullet train moves through a tunnel while the radio terminal is used in the train, the phase of the radio signal undergoes ultrafast polarity reversal due to the Doppler effect. The AFC of the radio base station therefore needs to have very high tracking capability so that such a radio signal can be properly received.
On the other hand, phase rotation of the received signal observed at the radio base station originates mostly from instability of the oscillator. Thus, where noise and the like attributable to instability of the oscillator is taken into consideration, the AFC needs to have high stability, rather than high tracking capability.
The tracking capability and stability of the AFC are in the trade-off relationship. Thus, if the tracking capability of the AFC is enhanced in order that the radio signals from radio terminals moving at high speeds can be received, the reception quality of radio signals from radio terminals moving at low speeds lowers. On the other hand, if the stability of the AFC is enhanced so as to improve the reception quality of radio signals from slow-moving radio terminals, then it is difficult to receive radio signals from fast-moving radio terminals.
In this manner, it has been difficult to improve both the reception quality of radio signals from fast-moving radio terminals and that of radio signals from slow-moving radio terminals.